Method of making a permanent magnet



METHOD on -r- A PERMANENT MAGNET No Drawing. Application April 18, 1955 Serial No. 502,237

Claims priority, application Netherlands April 20, 1954 Claims.

Our invention relates to a method of making permanent magnets and in particular to permanent antisotropic magnets.

Specifically our invention relates to the manufacture of permanent magnets which are constituted principally by hexagonal crystals having dimensions whose size approach that of the Weiss domains in the material, i. e. dimensions less than a, and which have a composition M0.6Fe O M being at least one of the metals of the group consisting of barium, strontium, and lead, with up to 40% by weight of said latter elements being replaceable by calcium.

In comparison with steel-alloy magnets which contain cobalt and nickel, the aforesaid magnets have a comparatively low remanence. Since the remanence of the magnet is directly related to its density, such magnets generally have a density of more than 4 and preferably between 4.6 and 5.0 which is most conductive to a permanent magnet of high density and high coercivity.

Another factor of importance in determining the quality of a permanent magnet is the (BH) value or energy product of the magnet. said type generally have a (BH) value not exceeding 1.l 10 gauss-oersted. Anistropic magnets of the aforesaid type can be made by orienting the magnetic material while in a relatively mobile state, i. e. by suspending the material in a magnetic field and forming a body under pressure while the material is subjected to the magnetic field. This body is then sintered at a temperature of about 1100 to 1400" C. to form a coherent dense body, i. e. a body having a density more than 4. Such anisotropic magnets may have a (Bl-l) substantially greater than 1.1)(10 gauss-oersted. 1 7

It is an object of our invention to make permanent anisotropic magnets of the aforesaid type having a (BH) likewise exceeding 1.1)(10 gauss-oersted.

It is a principal object of our invention to make perrnanent anisotropic magnets of the aforesaid type having a (BH) exceeding 2.0)(10 gauss-oersted.

It is a further object of our invention to simplify the manufacture of permanent anisotropic magnets of the aforesaid type.

It is a still further object of our invention to make permanent anisotropic magnets of the aforesaid type more economically.

These and further objects of our invention will appear as the specification progresses.

In the manufacture of permanent anisotropic magnets of the aforesaid type, three stages of preparation are required, namely,

(1) Preparation of an isotropic material by mixing and sintering together about 6 moles of Fe O and about one mole of one or more of the oxides of barium, strontium and lead. In this step, a material is formed which Isotropic magnets of the afore-.

2,837,483 Patented June 3, 1958 is ferromagnetic and has a composition MO.6Fe O M being one of the latter metals.

(2) Magnetic orientation of the isotropic material while molding the same into a body.

(3) Final sintering of the molded body at relatively high temperatures to form a dense product.

While basically our invention involves the same steps, we have found quite unexpectedly that in step 2, we can add up to by weight of the unrcacted oxides leading to the product in step 1 and the sintered body produced in step 3 will have a (BH) value still exceeding 1.1 10 gauss-oersted. Thus, only 20% by weight of the material in step 2 need be reacted oxides which effects a considerable simplification and economy in the procedure.

The unreacted oxides may be mixed with the isotropic material in a ratio other than their ratio in the reacted product.

If, to the material in step 2, before pressing, we add about 0.01 to- 1% by weight of modifier, i. e. an oxide such as calcium oxide, barium oxide, lanthanum oxide, bismuth oxide, arsenic trioxide, antimony pentoxide; or

a compound which decomposes upon heating toformf those oxides such as calcium carbonate, barium carbonate or barium sulphate; or a compound of one of those oxides and ferric oxides such as barium ferrite and calcium ferrite, products having (BH) values even higher, e. g. up to 3.0)(10 may be obtained.

In the following examples we have described procedures for making permanent anisotropic magnets in accordance with our invention which are to be considered illustrative only, the invention being defined in the claims appended to this specification.

EXAMPLEI A dry mixture of barium carbonate, BaCO and iron oxide, Fe O in the molecular ratio of 1:5.5 was ground in a vibration mill for three hours. This powder mixture is indicated in the following table by A. Part thereof was presintered by heating to about 1280 C. for about 30 minutes, cooled, suspended in water, ground in a ball mill for 25 hours and then dried and sieved. The, finely-comminuted presintered material thus obtained is indicated in the following table by B.

The powders A and B were mixed in diiferent ratios by grinding them together in a vibration mill for half an hour. A suspension of this material in water of about 60% by weightwas prepared and placed between the poles of a strong electro-rnagnet. The water was drained oif by compressing the suspension with the magnetic field applied to the suspension. The compressed body thus obtained was heated to a temperature of 1240 C. to 1250" C. for about 10 minutes. Table I shows the relationship between materials prepared by mixing powders A and B in different proportions and the characteristic properties of the permanent magnets obtained. By Way of explanation it should be noted that the orientation ratio BBL is a measure for the anisotropy of the magnets obtained. B H designates the remanence measured in a direction parallel to the external magnetic field applied during compression; B, designates the remanence measured in a direction at right angles thereto.

Table I Comp. of the compr. body Coercivity (BH) Or. ratio Rem. B, (B c) in in Gauss- Density Br" Percent Percent in Gauss Oersted Oersted I by w. by w.

100 3,050 2,020 2. 03 l0 4. 58 2.13 10 90 3, 050 1, 950 2. 03X10 4. 6G 2. 13 20 80 3, 120 1, 820 2. 04Xl0 4. 72 2. 18 40 60 3, 180 1, 800 2. 03X10 4. 69 2. 16 60 40 2, 840 1, 800 1. 65x10 4. 72 1. 77 80 20 2, 440 l, 710 1. 21 10 4. 65 l. 87 100 1, 900 1, 500 0. 75 10 4. 45 1.

EXAMPLE II iron oxide, Fe O in a molecular ratio of 1:5, was ground A dry mixture of barium carbonate, BaCQ and iron oxide Fe O in the molecular ratio of 1:5.5 was ground in a vibration mill for three hours. This powder mixture is indicated in thefollowing table by A. Part thereof was presintered by heating to about 1280 C. for about minutes and then cooled. About 0.5% by weight of calcium carbonate, CaCO was added to the presintered product which was ground in the presence of water for 24 hours and then dried and sieved. The finely-comminuted presintered product thus obtained is indicated in the following table by B. In the same manner as described in Example I, permanent magnets were made from a number of mixtures of the powders A and B in different proportions. Table II shows the relationship between the proportions of the powders A and B and the characteristic properties of the magnets obtained.

in a vibration mill for four hours. This powder mixture is indicated in the following Table III by A. Part thereof was presintered by heating to about 1280 C. for about 30 minutes; it was then cooled, ground in the presence of water in a ball mill for hours, dried and seived. The finely-comminuted presintered product thus obtained is indicated in the following Table III by B.

The powders A and B were mixed in different ratios by grinding them together in a vibrant mill for one hour. Then a quantity of water of about 60% by weight of the powder mixture was added and from the suspension obtained the water was drained by compression, while the suspension was disposed between the poles of a strong electromagnet. The compressed body thus obtained was heated to a temperature of about 1275 C. for about 10 minutes. Table III indicates the relationship be- Table 11 Comp. of the compr.

body Goereivity (1311).... Or. ratio Rem. B, (B c) in in Gauss- Density ,1] Percent Percent in Gauss Oersted Oersted BLL by w. by w.

.... 100 3, 690 1, 260 2. 53x10 6. 00 3. 80 10 90 3, 670 1, 300 2. 58X10 5. 03 3. 78 20 80 3, 640 1, 300 2. X 10' 4. 97 3. 96 40 60 3, 500 1, 370 2. 38X 10 4. 89 3. 71 60 40 3, 030 1, 620 1. 86 10 4. 63 2. 55 80 20 2, 450 1, 540 1. 14X10 4. 65 1. 46 100 1, 900 l, 500 0. 10 4. 41 1. 00

From the examples described above it is evident that: (a) with a ratio of 40% by weight of A to 60% by tween the proportions of the powders and the characteristic properties of the permanent magnets obtained.

Table III Comp. of the compr.

body

' Ooercivity (BH)m x Or. ratio Rem. B, (B c) in in Gauss- Density B, Percent Percent in Gauss Oersted Oersted by w. by w.

3,300 1,560 2 34x10 4.85 2.3 40 60 3, 620 1, 600 2 46x10 4. 66 3. 6 8G 20 3,160 1, 860 2 21 10 4.66 2.8 100 2, 1, 600 0 96 l0 4. 83 1. 0

weight of B a magnet was obtained which had a (BI-U EXAMPLE IV value which is not appreciably lower than that of a magnet obtained from a completely presintered powder (100% by weight of B); (1)) even in the presence of not more than 20% by weight of presintered powder in the compressed body a (BI-U value is obtained, which is materially higher than that of an isotropic magnet of the same material.

EXAMPLE III A dry mixture of strontium carbonate, SrCO and form tablets having a diameter of about 7 cms. and a thickness of 2 to 4 ems. These tablets were heated to a temperature of about 1280 C. for about 15 minutes. Subsequent to cooling the tablets were hammered to fine ness and ground dry for 15 minutes. Then the grinding operation was continued for 4 hours with alcohol as a grinding liquid in a vibration mill, after which the alcohol was evaporated. The finely comminuted presintered product thus obtained is indicated in the following Table IV by B.

In the same manner as indicated in Example I, permanent magnets are made from a number of mixtures of the powders A and B with difierent ratios of mixing. The temperature at which the compressed bodies were heated, was 1270 C.

Table IV shows the relationship between the proportions of the powders A and B and the characteristic properties of the permanent magnets obtained.

about 0.1 to 1% by weight of calcium carbonate as a modifier with up to four times the weight of said material of a mixture of an oxide of one of said metals and Fe O said mixture containing about 1 mole of an oxide M0 for each 6 moles of R3 0 magnetically orienting said permanent magnetic material in the mixture while the latter is in a mobile condition, and sintering said mixture at a temperature of about 1000 to 1400 C. to form a body having a density of at least 4 and constituted principally by hexagonal crystals having dimensions smaller than 10 and having a composition corresponding to said formula NIO.6F203.

4. A method of producing an anisotropic permanent magnet comprising the steps of mixing about 6 moles of ferric oxide and about one mole of an oxide of at least one metal selected from the group consisting of barium, strontium and lead, presintering said mixture at about 1100 to 1400 C. to form a product consisting principally Table IV Comp. of the eompr. body Coerelvity (311) 0r. ratio Rem, B, (B 0) in in Gauss- Density Percent Percent in Gauss Oersted Oersted ELL by w. by w.

100 3,760 1,800 2 86X10 4,95 3. 5 80 3, 660 1, 680 2 84X10 4. 77 3.8 40 60 3,380 1, 950 2 65X10 4. 64 3.2 60 40 3, 150 2, 300 2 20X10 4.37 2. 9 80 20 2,680 1,800 1 50x10 3. 94 2.2

What is claimed is:

1. A method of producing an anisotropic permanent magnet comprising the steps of mixing a finely comminuted permanent magnetic material having a composition MO.6Fe O M being a metal selected from the group consisting of barium, strontium and lead, with up to four times the weight of said material of a mixture of an oxide of one of said metals and Fe O said mixture containing about 1 mole of an oxide M0 for each 6 moles of Fe O magnetically orienting said permanent magnetic material in the mixture while the latter is in a mobile condition, and sintering said mixture at a temperature of about 1100 to 1400 C., to form a body having a density of at least 4 and constituted principally by hexagonal crystals having dimensions smaller than 10,11. and having a composition corresponding to said formula MO.6Fe-,,O

2. A method of producing an anisotropic permanent magnet comprising the steps of mixing a finely permanent magnetic comminuted material having a composition MO.6Fe O M being a metal selected from the group consisting of barium, strontium and lead, and about 0.6 to 1% by weight of a modifier selected from the group consisting of calcium oxide, barium oxide, lanthanum oxide, bismuth oxide, arsenic trioxide, antimony pentoxide, and compounds which form these oxides upon heat decomposition with up to four times the weight of said material of a mixture of an oxide of one of said metals and Fe O said mixture containing about 1 mole of an oxide M0 for each 6 moles of Fe O magnetically orienting said permanent magnetic material in the mixture while the latter is in a mobile condition, and sintering said mixture at a temperature of about 1100 to 1400 C. to form a body having a density of at least 4 and constituted principally by hexagonal crystals having dimensions smaller than 10 and having a composition corresponding to said formula MO.6Fe O 3. A method of producing an anisotropic permanent magnet comprising the steps of mixing a finely comminuted permanent magnetic material having a composition MO.6Fe O M being a metal selected from the group consisting of barium, strontium and lead, and

of a composition MO.6Fe O M being one of said metals, comminuting said product mixing said product with up to four times the weight of said material of a mixture of an oxide of one of said metals and Fe O said mixture containing about 1 mole of an oxide M0 for each 6 moles of Fe O magnetically orienting said permanent magnetic material in the mixture while the latter is in a mobile condition, and sintering said mixture at a temperature of about 1100 to 1400 C. to form a body having a density of at least 4 and constituted principally by hexagonal crystals having dimensions smaller than 10g and having a composition corresponding to said formula MO.6F6203.

5. A method of producing an anisotropic permanent magnet comprising the steps of mixing about 6 moles of Fe O and 1 mole of BaCO presintering said mixture at about 1100 to 1400 C. to form a product consisting principally of a composition BaO.6Fe- O comminuting said product, mixing said comminuted product with up to four times the weight of said material of a mixture of an oxide of at least one metal selected from the group consisting of barium, strontium, and lead, and Fe 0 said mixture containing about 1 mole of an oxide M0 for each 6 moles of Fe O M being one of said metals, magnetically orienting said permanent magnetic material in the mixture while the latter is in a mobile condition,

References Cited in the file of this patent UNITED STATES PATENTS Albers-Schonberg Jan. 18, 1955 OTHER REFERENCES Electrical Engineering, July 1952, article by Brockman, pages 644-647.

J. Applied Physics, page 1282, vol. 23, 1952.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, 2,837,483 June 3, 1958 Petrus Johannes Hakker et al,

It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, line 26, for 'vibrant" read vibration line 67, for oarniunw read barium column 6, line 8, for "1000 to 1400 Ca read w 1100 to 1400 on Signed and sealed this 26th day of August 1958o (SEAL) Attest:

KARL H, AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents 

1. A METHOD OF PRODUCING AN ANISOTROPIC PERMANENT MAGNET COMPRISING THE STEPS OF MIXING A FINELY COMMINUTED PERMANENT MAGNETIC MATERIAL HAVING A COMPOSITION MO6FE 2O3,M BEIN A METAL SELECTED FROM THE GROUP CONSISTING OF BARIUM, STRONTIUM AND LEAD, WITH UP TO FOUR TIMES THE WEIGHT OF SAID MATERIAL OF A MIXTURE CONOXIDE OF ONE OD SAID METALS AND FE2O3, SAID MIXTURE CONTAINING ABOUT 1 MOLE OF AN OXIDE MO FOR EACH 6 MOLES OF FE2O3 MAGNETICALLY ORIENTING SAID PERMANENT MAGNETIC MATERIAL IN THE MIXTURE WHILE THE LATTER IS IN A MOBILE CONDITION, AND SINTERING SAID MIXTURE AT A TEMPERATURE OF ABOUT 1100 TO 1400*C., TO FORM A BODY HAVING A DENSITY OF AT LEAST 4 AND CONSTITUTED PRINCIPALLY BY HEXAGONAL CRYSTALS HAVING DIMENSIONS SMALLER THAN 10U AND HAVING A COMPOSITION CORRESPONDING TO SAID FORMULA MO.6FE2O3. 